6xxxx-series rolled sheet product with improved formability

ABSTRACT

A rolled 6xxx-series aluminium alloy sheet product including, in wt. %: Mg 0.10-1.3%, Si 0.25-1.4%, Cu up to 0.45%, Mn up to 0.50%, Fe 0.10-0.45%, Sr 0.01-0.05%, and wherein the ratio Fe/Sr is in a range of 4 to 40, Ti up to 0.20%, Cr up to 0.30%, Zr up to 0.20%, V up to 0.20%, Zn up to 0.35%, Sn up to 0.075%, impurities and balance aluminium. A method of manufacturing the 6xxx-series aluminium alloy sheet product.

FIELD OF THE INVENTION

The invention relates to a rolled 6xxx-series aluminium alloy sheetproduct. The sheet product is ideally suitable for automotiveapplications. The invention further relates to a method of manufacturingthe 6xxx-series aluminium alloy sheet product.

BACKGROUND TO THE INVENTION

Generally, body panels of a vehicle require excellent properties informability, dent-resistance, paint-bake response, corrosion resistanceand surface quality properties. However, the conventional AA5xxx-seriesalloy sheets have not been favoured because they have low mechanicalstrength even after press forming and may also exhibit poor surfacequality. Therefore, 6xxx-series sheet alloys have been increasinglyused. In general, the 6xxx-series alloys provide excellent bakeharden-ability after painting and high mechanical strength as a result,thus making it possible to manufacture more thin-gauged and morelight-weight sheets in combination with a good corrosion resistance andclass-A surface finish. There is a need for aluminium alloy rolled sheetproducts suitable for use in automotive panels and exhibiting improvedformability.

DESCRIPTION OF THE INVENTION

As will be appreciated herein below, except as otherwise indicated,aluminium alloy designations and temper designations refer to theAluminium Association designations in Aluminium Standards and Data andthe Registration Records, as published by the Aluminium Association in2016 and are well known to the person skilled in the art.

For any description of alloy compositions or preferred alloycompositions, all references to percentages are by weight percent unlessotherwise indicated. The term “up to” and “up to about”, as employedherein, explicitly includes, but is not limited to, the possibility ofzero weight-percent of the particular alloying component to which itrefers. For example, up to 0.35% Zn may include an alloy having no Zn,and thus there may be an absence of such element.

It is an object of the invention to provide a 6xxx-series aluminiumalloy sheet product with improved formability.

This and other objects and further advantages are met or exceeded by thepresent invention and providing a rolled 6xxx-series aluminium alloysheet product consisting of, in wt. %:

Mg 0.10% to 1.3%, Si 0.25% to 1.4%, Cu up to 0.45%, Mn up to 0.50%, Fe0.10% to 0.45%, Sr 0.01% to 0.05%,

and wherein the ratio Fe/Sr is in a range of 4 to 40,

Ti up to 0.20%, Cr up to 0.30%, Zr up to 0.20%, V up to 0.20%, Zn up to0.35%, Sn up to 0.075%,

other elements and impurities each <0.05%, total <0.15%, balancealuminium.

In accordance with the invention, it has been found that the purposiveaddition of strontium (Sr) in the defined range in combination with theFe/Sr ratio offers an improved formability, in particular an increasedstretch formability assessed by an Erichsen Dome Height test. Inaddition, the rolled sheet product in accordance with the inventionallows for the presence of higher amounts of Fe while offering aformability commonly found in 6xxx-series alloy sheet products having alower Fe-content. This enables to maintain balance of strength and agood formability while using a higher content of recycled material andthereby increasing the environmental sustainability.

Some prior art documents disclosing the addition of strontium (Sr) towrought aluminium alloys are:

Patent document WO 2005/108633 A2 (Erbslöh) discloses 6xxx-seriesaluminium alloy having 0.3-0.9% Si, 0.1-0.5% Mg, up to 0.2% Fe, 0.1-0.4%Cu, 0.03-0.2% Mn, 0.01% Ti, 0.08-0.22% Zr and/or Cr and/or V, up to0.005% Ag, up to 0.04% Zn, and wherein the ratio (in wt. %) of Si to Mgis 1.8:1 to 3.3:1, and the ratio (in wt. %) of Fe to Sr is 3:1 to 5:1.The strontium addition is to ensure that the alloy can be decorativelyanodized and shows no yellowish or cloudy eloxal coating. The strontiumis said to alter the Fe-, Zr-, Cr, and/or Fe-containing phases to theextent that they do not cause visible clouding when they areincorporated in the eloxal coating.

Patent document U.S. Pat. No. 3,926,690 (Alcan) discloses that theaddition of 0.02-0.05% of Sr and/or Ca to the AA6063 extrusion alloy inorder to promote the formation of the less detrimental alpha-AlFeSiform, having the effect of improving the surface quality of theextrusion at increased extrusion speeds. No effects on mechanicalproperties are reported.

The paper “Effect of strontium on microstructure and properties ofaluminium based extrusion alloy 6061”, by F. Paray et al., in MaterialsScience and Technology, April 1996, Vol. 12, pp. 315-322 shown that inthis extrusion alloy, the strontium altered the platelike ß-AlFeSi phase(Al₅FeSi) to the Chinese script alpha-AlFeSi compound (Al₈Fe₂Si). Whilestrontium may shorten the homogenisation process, it has no adverseeffects on the mechanical properties of the extruded end products; aslight decrease in tensile strength of the strontium containing alloywas observed.

Effects and reasons for limitations of the alloying elements in theAl—Mg—Si alloy sheet manufactured in accordance with the method of thepresent invention are described below.

The purposive addition of Mg and Si strengthens the alloy due toprecipitation hardening of elemental Si and Mg₂Si formed under theco-presence of Mg. In order to provide a sufficient strength level inthe sheet product according to the invention, the Si content should beat least 0.25%, and preferably at least 0.50%, and more preferably atleast 0.65%. In an embodiment, the Si content is at least 0.75%. Apreferred upper-limit for the Si content is 1.4%, and more preferably1.3%. The presence of Si enhances also the formability, and excess Siwith respect to Mg promotes a fast paint-bake response.

Substantially for the same reason as for the Si content, the Mg contentshould be at least 0.10%, and preferably at least 0.20%, and morepreferably at least 0.25% to provide sufficient strength to the sheetproduct. The upper-limit for the Mg content is 1.3%, and preferably1.0%. A too high Mg excess may increase the fraction of undesired secondphase particles by the formation of Al—Fe—Mg phases.

In one embodiment, the rolled 6xxx-series aluminium alloy sheet producthas a ratio (in wt. %) of Si/Mg of at least 0.90. Preferably, the Si/Mgratio does not exceed 1.40, and more preferably it does not exceed 1.30.This embodiment has also a very good corrosion resistance and highbendability and hemmability. In combination with the addition of Sr, asignificant improved Erichsen Dome Height is achieved.

In one embodiment, the rolled 6xxx-series aluminium alloy sheet producthas a ratio (in wt. %) of Si/Mg in a range of 2.0 to 7.0. A preferredlower-limit for the Si/Mg ratio is 2.5, and more preferably 3.0, andmore preferably 4.0. A preferred upper-limit for the Si/Mg ratio is 6.5.This embodiment has in particular a very good formability, more inparticular a good Erichsen Dome Height is achieved.

In the embodiment wherein the rolled 6xxx-series aluminium alloy sheetproduct has a ratio (in wt. %) of Si/Mg in a range of 2.0 to 7.0, andwith preferred narrow ranges, to provide improved formability, it ispreferred that the area fraction of Si-particles having a size of morethan 0.35 microns (when observed by Light Optical Microscopy at amagnification of 500×) is less than 0.15%, and preferably less than0.11%. In a further embodiment, the equivalent average radius ofSi-particles having a size of more than 0.35 microns (when observed byLight Optical Microscopy at a magnification of 500×) is less than 1.4microns, and preferably less than 1.3 microns.

Copper (Cu) can be present in the rolled 6xxx-series sheet product toenhance in particular the work hardening behaviour and the paint-bakeresponse, but it should not exceed 0.45%. A preferred upper-limit forthe Cu-content is 0.40%, and more preferably 0.30%. In a preferredembodiment, Cu is purposively added in a range of at least 0.02%, andpreferably of at least 0.04%.

Iron (Fe) should remain within a range of 0.10% to 0.45%. A too lowFe-content may lead to undesired grain growth in the final sheet productadversely affecting several formability characteristics. In addition,the effectivity of the Sr addition is less at a low Fe content. A toohigh Fe-content has an adverse effect on the formability and mechanicalproperties and is difficult to compensate by the purposive addition ofSr. A preferred lower-limit for the Fe-content is 0.18%. In anembodiment, the Fe-content is at least 0.20%. In an embodiment, theFe-content is at least 0.22%. A preferred upper-limit for the Fe-contentis 0.40%, and more preferably 0.35%.

The strontium (Sr) content must be within a range of 0.01% and 0.05%,and furthermore, the ratio (in wt. %) of Fe/Sr is in a range of 4 to 40.A preferred upper-limit for the Sr content is 0.045%, and morepreferably 0.04%. A preferred lower-limit for the Sr-content is 0.02%,and more preferably 0.025%.

The purposive addition of Sr decreases the area fraction, the numberdensity, and the size (circular equivalent average radius) of coarse Siparticles (>0.35 micron), and also the area fraction, the numberdensity, and the size (circular equivalent average radius) of coarseMg₂Si particles (>0.35 micron). The addition of Sr decreases the grainsize without increasing the size of the fine second phase particles(these are particles <1 micron diameter that give contrast in LightOptical Microscopy at a magnification of 1000×) and of Fe-bearingparticles. This contributes to an increased formability of the6xxx-series aluminium sheet material, in particular the stretchformability as indicated, for example, by the Erichsen Dome Height andalso increases the elongation.

A preferred lower-limit for the Fe/Sr ratio is 5, and a more preferredlower limit is 6. A preferred upper-limit for the Fe/Sr ratio is 20, andmore preferably 15. Any Sr is preferable added to the aluminium alloy inthe form of a master-alloy, e.g. AlSr3.5 or AlSr5 or AlSr10, prior tocasting of the 6xxx-series alloy into rolling feedstock. Strontium isnot a common alloying element in rolled 6xxx-series aluminium alloyproducts, and consequently, the Sr level in any scrap thereof is verylow. Often, the Sr level is not standard measured for scrap material,and if present, the Sr level is commonly well below 0.005%, and moretypically below 0.001%.

Each of Mn, Cr, V, and Zr could be present to control the grain size inthe rolled aluminium alloy sheet product.

In a preferred embodiment, at least Mn is present in a range of 0.02% to0.50%. A preferred lower-limit for the Mn content is about 0.05%. Apreferred upper-limit for the Mn content is about 0.20%, and morepreferably 0.15%, and more preferably 0.10%. Mn is added for grain sizecontrol. A too high addition of Mn may interfere with the positiveaction of the Sr addition.

In a preferred embodiment, there is a purposive addition of Cr in arange of 0.01% to 0.30%. A preferred upper-limit for the Cr addition isabout 0.25%, and more preferably about 0.20%.

In a preferred embodiment, there is a purposive addition of at least Mnin combination with Cr.

Also, each of vanadium (V) and zirconium (Zr), each up to 0.20%, can beadded to control the grain size in the final sheet product. In apreferred embodiment, these are preferentially avoided in the rolledaluminium alloy sheet product as they may prevent full recrystallizationof the sheet product. Such elements are costly and/or form detrimentalintermetallic particles in the aluminium alloy. Thus, the rolledaluminium alloy sheet product generally includes not greater than 0.03%V and not greater than 0.03% Zr. In a preferred embodiment, the sheetproduct includes V only up to 0.02%, and more preferably up to 0.005%.In a preferred embodiment, the sheet product includes Zr only up to0.02%, and more preferably only up to 0.01%.

Zn is an impurity element that can be tolerated up to about 0.35% and ispreferably as low as possible, for example 0.20% or less, and morepreferably 0.10% or less.

The addition of tin (Sn) may assist in stabilising the mechanicalproperties in T4 temper. When added, a preferred addition of Sn is in arange of 0.005% to 0.075%, and more preferably in a range of 0.01% to0.06%.

Ti can be added to the sheet product amongst others for grain refinerpurposes during casting of the alloy ingots. The addition of Ti shouldnot exceed about 0.20%, and preferably, it should not exceed about0.10%. A preferred lower limit for the Ti addition is about 0.01%, andtypically, a preferred upper-limit for Ti is about 0.05%. The Ti can beadded as a sole element or with either boron or carbon serving as acasting aid for grain size control.

Unavoidable impurities can be present up to 0.05% each, and a total of0.15%, the balance is made with aluminium.

In the rolled 6xxx-series aluminium alloy product, there is no purposiveaddition of elements like In, Th, Er, Sb, Hf, La, Ce, Sm. These areunusual impurities in 6xxx-series alloys, and preferably for each ofthese elements their presence, if any, is up to 0.005% maximum.

In an embodiment, the rolled 6xxx-series aluminium alloy product in a T4temper has a tensile strength (R_(m)) of 200 MPa or more and a yieldpoint (R_(p0.2)) of 90 MPa or more when measured within 30 days aftersolution heat treatment and quench. In an embodiment, the yield point(R_(p0.2)) is less than 130 MPa after 6 months storage at ambient (room)temperature. In a further embodiment, it has an elongation at break(A_(80mm)) of at least 24%, and a uniform elongation (A_(g)) of at least20%.

In an embodiment, the rolled 6xxx-series aluminium alloy product in a T4condition and having a sheet material thickness of 1 mm has an ErichsenDome Height of at least 9.0 mm, and preferably of at least 9.2 mm, whentested in accordance with EN ISO 20482 (July 2003).

The 6xxx-series aluminium alloy according to this invention can beprovided as an ingot or slab for fabrication into rolling feedstockusing semi-continuous casting techniques regular in the art for castproducts, e.g. direct chill DC-casting and electro-magnetic EMC-casting,and preferably having an ingot thickness in a range of about 220 mm ormore, e.g. 400 mm, 500 mm or 600 mm. In another embodiment, thin gaugeslabs resulting from continuous casting, e.g. belt casters or rollcasters, also may be used, and having a thickness of up to about 40 mm.

After casting the rolling feedstock, the thick semi-continuous as-castingot is commonly scalped to remove segregation zones near the castsurface of the ingot.

Next homogenisation should be performed at a temperature of 450° C. ormore. If the homogenisation temperature is less than 450° C., reductionof ingot segregation and heterogeneity may be insufficient. This resultsin insufficient dissolution of Mg₂Si components which contribute to thestrength, and whereby formability may be decreased. Homogenisation ispreferably performed at a temperature of 480° C. or more, morepreferably at least one homogenisation step is performed at atemperature range of 540° C. to 580° C. The heat-up rates that can beapplied are those which are regular in the art.

The soaking times for homogenisation should be at least about 2 hours,and more preferably at least about 5 hours. A preferred upper-limit forthe homogenisation soaking time is about 48 hours and more preferablyabout 24 hours.

The hot rolling practice comprises a first hot rolling operation whereinthe heated feedstock is subjected to breakdown hot rolling in one ormore passes using reversing or non-reversing mill stands that serve toreduce the thickness of the rolling feedstock or ingot to anintermediate gauge range of 15 mm to 40 mm, and preferably of 15 to 35mm. The breakdown rolling starts preferably at a temperature in therange of about 460° C. to 510° C., and more preferably of 470° C. to500° C. The hot-mill process temperature should be controlled such thatafter the last rolling pass the hot-mill exit temperature of thefeedstock is in a range of about 370° C. to 480° C. A more preferredlower-limit is about 380° C. A more preferred upper-limit is about 450°C., and more preferably 430° C.

Next after the breakdown hot rolling, the feedstock is supplied to amill for hot finish rolling in one or more passes to a final gauge inthe range of 3 mm to 15 mm, for example, 7 mm or 10 mm. The hotfinishing rolling operation can be done, for example, using a reversemill or a tandem mill. Overall, the thickness of the rolling feedstockor ingot is typically reduced by at least about 65%, and more typicallyin the range of 80% to 97%. The average temperature of the hot rolledfeedstock when the feedstock is inputted into the hot finish rollingprocess is maintained preferably at a temperature of 370° C. to 480° C.A more preferred lower-limit is about 400° C. A more preferredupper-limit is about 450° C.

Control of the finish hot-mill exit temperature of the rolling feedstockis important to arrive at the desired balance of metallurgicalproperties. The hot-mill temperature should be controlled such thatafter the last rolling pass the hot-mill exit temperature of thefeedstock is in a range of about 300° C. to 400° C. A preferredlower-limit is about 310° C. A preferred upper-limit is about 380° C.,and more preferably about 360° C. A too low or too high exit-temperatureof the hot rolled feedstock adversely affects the formability propertiesof the final product.

Following the last hot-rolling step, the hot-rolled feedstock at finalgauge is cooled to below 200° C., more typically to below 100° C., andpreferably to ambient temperature. In a preferred embodiment, thecooling of the hot-rolled feedstock at final gauge from hot-mill exittemperature is by the immediately coiling of the hot-rolled feedstockand allowing it to cool in an ambient environment to ambient temperatureand stored.

In a next step, the hot rolled material is further down gauged by coldrolling to final gauge by applying in one or more rolling steps a totalcold rolling degree of at least 40%, preferably of at least 60%.

Optionally during the cold rolling operation, a recrystallizationannealing (continuous or batch) can be applied to the cold-rolledproduct at an intermediate gauge. The annealing temperature is in therange of 360° C. to 580° C. to achieve recrystalisation in thecold-rolled product which influences the crystallographic texturedevelopment. A preferred lower-limit for the annealing temperature is380° C., and more preferably 400° C. A preferred upper-limit for theannealing temperature is 500° C., and more preferably 460° C.

Following the optional intermediate annealing heat treatment, thefeedstock is cold rolled in one or more cold rolling steps to a finalgauge in a range of 0.7 mm to 4.0 mm. A preferred upper-limit for thesheet thickness is 3.0 mm and more preferably 2.5 mm.

In an embodiment of the method, the cold rolled aluminium sheet productat final gauge is solution heat treated at a temperature and for a timesuch that substantial amounts of Mg₂Si and Si, if any, are dissolvedinto solid solution. The solution heat-treatment temperature is at least500° C., and is preferably in a range of 520° C. to 570° C., and morepreferably in the range of 530° C. to 565° C., and is more preferablyjust above the solvus temperature of the Mg₂Si and Si phases to furtherimprove formability characteristics of the aluminium alloy sheetproduct. After the solution heat treating, the sheet is quenched, e.g.,by means of water such as cold water quenching or cold water sprayquenching. By these processing steps, the main alloying elements Mg, Siand Cu are mostly dissolved during SHT and retained in solid solution bythe quenching operation leading to a good formability and control of theyield strength and the bake hardening behaviour. The evolution of themicrostructure at ambient (room) temperature brings the sheet materialfrom a W (as quenched) to a T4 condition.

In an embodiment, following the solution heat treatment and quenching ofthe sheet product, the sheet product is subjected to artificial ageingor pre-ageing and then natural ageing for 72 hours or longer prior toforming into, e.g., a three-dimensional shaped or formed automotive bodymember. The pre-ageing is preferably performed in a continuous annealingline immediately following the solution heat treatment and quenching byheating up to a temperature in a range of 50° C. to 130° C. Thepre-ageing treatment provides in time more stable mechanical propertiesof the sheet product before and after being subjected to a paint bakecycle as well an increased paint-bake response.

In an embodiment, following the solution heat treatment and quenching ofthe sheet product, the sheet product is subjected to natural ageing for72 hours to 6 months, optionally even longer, prior to shaping orforming into, e.g., a three-dimensional shaped or formed automotive bodymember.

Forming operations into three-dimensional shapes includes deep-drawing,pressing, stamping, and stretch forming.

Following the forming operation, the formed part may be made part of anassembly of other metal components as regular in the art formanufacturing vehicle components, and subjected to a paint bakeoperation to cure any paint or lacquer layer applied. The paint bakeoperation or cycle comprises one or more sequential short heat treatmentin the range of 140° C. to 210° C. for a period of 10 to less than 40minutes, and typically of less than 30 minutes. A typical paint bakecycle would comprise a first heat treatment of 180° C.@20 minutes,cooling to ambient temperature, then 160° C.@20 minutes and cooling toambient temperature. In dependence of the OEM, such a paint bake cyclemay comprise of 2 to 5 sequential steps and includes drying steps.

In an embodiment, the rolled 6xxx-series aluminium sheet productsaccording to this invention are cast via continuous casting, e.g., beltcasters or roll casters, and have a feedstock thickness of up to about40 mm. Downstream of the continuous casting operation, the product canbe rolled (hot and/or cold), optionally annealed (e.g., between hotrolling and any cold rolling steps), solution heat treated and quenched,optionally cold worked (post-solution heat treatment) or natural agedand optionally also artificially aged, and all these steps may occurin-line or off-line relative to the continuous casting step. Theartificially aged product can be painted (e.g., for an automobile part),and may thus be subjected to a paint-bake cycle.

The rolled 6xxx-series aluminium alloy sheet product according to thisinvention is ideally suitable for manufacturing formed automotive bodymembers. A formed automotive body member includes bumpers, doors, hoods,roofs, trunk lids, fenders, floors, wheels and other portions of anautomotive or vehicle body such as body-in-white (e.g., pillars,reinforcements) applications. Due to its combination of excellent deepdrawing and stretch forming properties, the rolled 6xxx-series aluminiumalloy sheet product is also perfectly suited to produce also inner doorpanels, wheel arch inner panels, and side panels, spare wheel carrierpanels, and similar panels with a high deep drawing height.

The invention will now be illustrated with reference to non-limitingembodiments according to the invention.

Example 1

Six ingots having the dimensions of 430 mm×140 mm×2000 mm have beenDC-cast and the six alloy compositions are listed in Table 1, andwhereby alloys 3 and 6 are according to the present invention and havingan Fe/Sr-ratio of respectively 6.8 and 7.5.

Each ingot has been homogenised for 10 hours at about 560° C. and hotrolled from 80 mm to 10 mm. The hot-mill entry temperature was about550° C. Following hot rolling warm coiling and self-annealing wassimulated in a furnace. Next, the sheet products were cold rolled from10 mm to 3.0 mm, followed by a batch interannealing at 380° C.@2 hrs,and then cold rolled to a final gauge of 1.0 mm. At final gauge, thesheet products have been solution heat-treated at 560° C. for about 1minute and then cold water quenched to room temperature.

Several mechanical properties have been determined after 1 month naturalageing at room temperature (T4 temper) and in a T64 temper (1 monthnatural ageing followed by simulated paint-bake cycle of 2% pre-stainand 185° C.@20 minutes) to assess the paint-bake response of the sheetproducts.

The mechanical properties (yield strength Rp0.2, tensile strength Rm,uniform elongation Ag, elongation at fracture A80, strain hardeningexponent n, r90°-value) have been assessed in accordance withinternational standard ISO 6892-1 (second edition, July 2016), and inTable 2, the average over three measurements per sample are listed.

Further, the Erichsen Dome Height (EDH) has been measured in accordancewith EN ISO 20482 (July 2003). The EDH is used to assess the stretchformability of the sheet products in terms in plane stress biaxialtensile deformation. In Table 2, the average over three measurements persample are listed.

TABLE 1 Alloy composition of the aluminium alloys cast, the balance ismade by aluminium and unavoidable impurities. Alloying element (in wt.%) Alloy Mg Si Fe Mn Ti Sr 1 0.30 1.35 0.03 0.05 0.02 — 2 0.31 1.30 0.250.04 0.02 — 3 0.31 1.36 0.24 0.04 0.02 0.035 4 0.60 0.61 0.04 0.05 0.02— 5 0.59 0.61 0.25 0.05 0.02 — 6 0.59 0.60 0.24 0.05 0.02 0.032

TABLE 2 Mechanical properties in T4 and T64 temper. T4 temper T64 temperR_(p0.2) R_(m) A_(g) A₈₀ n- r90°- EDH R_(p0.2) R_(m) Alloy [MPa] [MPa][%] [%] value value [mm] [MPa] [MPa] 1 100.0 203.7 22.8 24.3 0.31 0.568.61 139.3 206.0 2 105.0 214.0 23.5 27.3 0.30 0.60 8.95 141.3 219.3 3105.3 218.7 22.9 26.3 0.30 0.59 9.24 143.3 228.7 4 97.0 193.7 22.9 25.00.30 0.61 8.85 147.0 206.3 5 93.3 199.0 21.7 24.8 0.29 0.70 8.99 150.7216.7 6 97.8 199.7 21.0 23.6 0.29 0.68 9.17 154.7 215.3

From the results of Table 2, it can be seen that an increased Fe-level(alloy 1 vs. alloy 2 and alloy 4 vs. alloy 5) increases the strength ofthe sheet product in T4 temper. Also, an increase in r90°-value andelongation and a decrease in n-value can be observed. These effects arebelieved to be due to the change of the grain size as observed by theaddition of Fe. From the comparison, an increase in EDH can also beobserved. The purposive addition of Sr (e.g., alloy 2 vs. alloy 3 andalloy 5 vs. alloy 6) further increases the strength in T4 temper, inparticular, the ultimate tensile strength. In particular, an furtherincrease in EDH can also be observed (e.g., alloy 2 vs. alloy 3 andalloy 5 vs. alloy 6).

This improvement in formability, in particular in stretch formabilityassessed in an EDH test, at increasing Fe levels shows that the rolled6xxx-series aluminium alloy sheet products in accordance with theinvention are ideal candidates for forming into complex automotivemembers, in particular when applying forming techniques requiring betterstretch formability. The capacity for scrap absorbing ensures a moreeconomical effective and environmentally friendly production of suchshaped complex automotive members.

Example 2

On an industrial scale of production, two rolling ingots having athickness of about 455 mm after scalping have been produced. The alloycompositions are listed in Table 3, and whereby alloy no. 8 is an alloyaccording to the invention and has an Fe/Sr ratio of 10.

Each ingot has been homogenised for 9 hours at about 560° C. and hotrolled to 7.5 mm. The hot-mill entry temperature was about 490° C. Next,the sheet products were cold rolled to 3 mm, followed by a batchinterannealing at 380° C.@2 hrs, and then cold rolled to a final gaugeof 1.0 mm. At final gauge, the sheet products have been solutionheat-treated at 565° C. and then cold water quenched to roomtemperature.

TABLE 3 Alloy composition of the aluminium alloys cast, the balance ismade by aluminium and unavoidable impurities. Alloying element (in wt.%) Alloy Mg Si Fe Mn Ti Sr 7 0.24 1.29 0.17 0.05 0.02 — 8 0.23 1.27 0.230.05 0.02 0.023

Several mechanical properties have been determined after 3 monthsnatural ageing at room temperature (T4 temper) and in a T64 temper (3months natural ageing followed by simulated paint-bake cycle of 2%pre-stain and 185° C.@20 minutes) to assess the paint-bake response ofthe sheet products.

The mechanical properties (yield strength Rp0.2, tensile strength Rm,uniform elongation Ag, elongation at fracture A80) have been assessed inaccordance with international standard ISO 6892-1 (second edition, July2016), and in Table 4, the average over three measurements per sampleare listed.

The Erichsen Dome Height (EDH) has also been measured in accordance withEN ISO 20482 (July 2003). The EDH is used to assess the stretchformability of the sheet products in terms in plane stress biaxialtensile deformation. In Table 4, the average over three measurements persample are listed.

TABLE 4 Mechanical properties in T4 and T64 temper. T4 T64 R_(p0.2)R_(m) A_(g) A₈₀ EDH R_(p0.2) R_(m) Alloy [MPa] [MPa] [%] [%] [mm] [MPa][MPa] 7 81.5 173 24.1 27.3 8.97 128.8 194.5 8 85.4 180 23.9 27.4 9.25133.4 201.6

From the results of Table 4, it can be seen that the purposive additionof Sr results in an improved strength both in T4 and T64 condition. Theformability by reference to the EDH results is also improved.

Samples of alloy 7 and 8 in the T4 condition have been analysed forseveral microstructural features, in particular the average grain sizeusing standard light optical microscopy techniques at a magnification of100×. The particle distribution of coarse Si particles larger than 0.35μm has been analysed using standard light optical microscopy techniquesat a magnification of 500×, and the results are listed in Table 5.Further, the Mg₂Si particles distribution larger than 0.35 μm has beenanalysed using SEM at a magnification of 500×, and the results arelisted in Table 6.

The average grain size in the RD direction through thickness for alloy 7was 58.2 μm and for alloy 8 it was 50.2 μm. The average grain size inthe ND direction through thickness for alloy 7 was 58.2 μm and for alloy8 it was 39.5 μm.

TABLE 5 Microstructural feature: the coarse Si particles (>0.35 μm)distribution Si particles (>0.35 μm) distribution Number densityCircular equivalent Alloy Area fraction (%) per square μm average radius(μm) 7 0.17 0.00019 1.7 8 0.07 0.00014 1.2

TABLE 6 Microstructural feature: the Mg2Si (>0.35 μm) particlesdistribution Mg2Si particles (>0.35 μm) distribution Number densityCircular equivalent Alloy Area fraction (%) per square μm average radius(μm) 7 0.07 0.00068 0.57 8 0.027 0.00032 0.52

From these results, it can be seen that the purposive addition of Srdecreases the area fraction, the number density, and the size (circularequivalent average radius) of coarse Si particles, and also the areafraction, the number density and the size (circular equivalent averageradius) of coarse Mg₂Si particles. The addition of Sr decreases also theaverage grain size. This contributes to an increased formability of the6xxx-series aluminium sheet material, in particular the stretchformability as indicated, for example, by the Erichsen Dome Height testresults and increases also the elongation.

The invention is not limited to the embodiments described before, whichmay be varied widely within the scope of the invention as defined by theappending claims.

1. A rolled 6xxx-series aluminium alloy sheet product consisting of analuminum alloy consisting of, in wt. %: Mg 0.10% to 1.3%, Si 0.25% to1.4%, Cu up to 0.45%, Mn up to 0.50%, Fe 0.10% to 0.45%, Sr 0.01% to0.05%,

and wherein the ratio Fe/Sr is in a range of 4 to 40, Ti up to 0.20%, Crup to 0.30%, Zr up to 0.20%, V up to 0.20%, Zn up to 0.35%, Sn up to0.075%,

other elements and impurities each <0.05%, total <0.15%, balancealuminium.
 2. The rolled 6xxx-series aluminium alloy sheet productaccording to claim 1, wherein the Si-content is at least 0.50%.
 3. Therolled 6xxx-series aluminium alloy sheet product according to claim 1,wherein the Mg-content is less than 1.3%.
 4. The rolled 6xxx-seriesaluminium alloy sheet product according to claim 1, wherein the ratioSi/Mg is at least 0.90.
 5. The rolled 6xxx-series aluminium alloy sheetproduct according to claim 1, wherein the ratio Si/Mg is in a range of0.90 to 1.40.
 6. The rolled 6xxx-series aluminium alloy sheet productaccording to claim 1, wherein the ratio of Si/Mg is in a range of 2.0 to7.0.
 7. The rolled 6xxx-series aluminium alloy sheet product accordingto claim 1, wherein the Fe/Sr ratio is in a range of 4 to
 40. 8. Therolled 6xxx-series aluminium alloy sheet product according to claim 1,wherein the Mn-content is in a range of 0.02% to 0.50%.
 9. The rolled6xxx-series aluminium alloy sheet product according to claim 1, whereinthe Cr-content is in a range of 0.01% to 0.30%.
 10. The rolled6xxx-series aluminium alloy sheet product according to claim 1, whereinthe Cu-content is at least 0.02%.
 11. The rolled 6xxx-series aluminiumalloy sheet product according to claim 1, wherein the Cu-content at most0.40%.
 12. The rolled 6xxx-series aluminium alloy sheet productaccording to claim 1, wherein the Fe-content at least 0.18%.
 13. Therolled 6xxx-series aluminium alloy sheet product according to claim 1,wherein the aluminium alloy sheet product has a gauge in the range of0.7 mm to 4.0 mm.
 14. The rolled 6xxx-series aluminium alloy sheetproduct according to claim 1, wherein said sheet product is provided inthe form of a shaped three-dimensional automotive body member.
 15. Amethod of manufacturing a rolled 6xxx-series aluminium alloy sheetproduct according to claim 1, wherein the method comprises the steps of:(a) semi-continuously or continuously casting an ingot of the aluminiumalloy; (b) homogenising of the ingot; (c) rolling of the ingot into analuminium alloy rolled sheet product having a final gauge in a range of0.7 mm to 4.0 mm; (d) solution heat treating the aluminium alloy sheetproduct at a temperature of at least 500° C.; (e) after the solutionheat treating, quenching of the aluminium alloy sheet product; and (f)natural ageing of the quenched aluminium alloy sheet product.
 16. Themethod according to claim 15, further comprising artificially ageing ofthe solution heat treated and quenched rolled sheet product.
 17. Amethod of manufacturing a rolled 6xxx-series aluminium alloy sheetproduct according to claim 1, the method comprising: continuouslycasting an ingot of the aluminium alloy; homogenising of the ingot;rolling the aluminium alloy into an aluminium alloy rolled sheet producthaving a final gauge in the range of 0.7 to 4.0 mm; solution heattreating the aluminium alloy sheet product at a temperature of at least500° C.; after the solution heat treating, quenching of the aluminiumalloy sheet product; and natural ageing of the quenched aluminium alloysheet product.
 18. The method of manufacturing a rolled 6xxx-seriesaluminium alloy sheet product according to claim 15, wherein solutionheat treating the aluminium alloy sheet product is at a temperature in arange of 520° C. to 570° C.
 19. The method of manufacturing a rolled6xxx-series aluminium alloy sheet product according to claim 15, whereinthe solution heat treated and quenched aluminium alloy sheet product hasbeen pre-aged.
 20. The method of manufacturing a rolled 6xxx-seriesaluminium alloy sheet product according to claim 15, wherein the naturalageing of the quenched aluminium alloy sheet product is for at least 72hours.
 21. The method of manufacturing a rolled 6xxx-series aluminiumalloy sheet product according to claim 15, wherein the method furthercomprising the step of shaping of the 6xxx-series aluminium alloy sheetproduct into a three-dimensional form.
 22. The method of manufacturing arolled 6xxx-series aluminium alloy sheet product according to claim 21,wherein the 6xxx-series aluminium alloy sheet product in athree-dimensional form is subjected to a paint-bake cycle.
 23. A methodof use of a rolled 6xxx-series aluminium alloy sheet product accordingto claim 1 comprising forming the sheet into a shaped three-dimensionalautomotive body member.
 24. The rolled 6xxx-series aluminium alloy sheetproduct according to claim 1, wherein the Si-content is at least 0.65%.25. The rolled 6xxx-series aluminium alloy sheet product according toclaim 1, wherein the Mg-content is less than 1.0%.
 26. The rolled6xxx-series aluminium alloy sheet product according to claim 1, whereinthe ratio of Si/Mg is in a range of 2.5 to 7.0.
 27. The rolled6xxx-series aluminium alloy sheet product according to claim 1, whereinthe Fe/Sr ratio is in a range of 5 to
 40. 28. The rolled 6xxx-seriesaluminium alloy sheet product according to claim 1, wherein the Fe/Srratio is in a range of 6 to
 40. 29. The rolled 6xxx-series aluminiumalloy sheet product according to claim 1, wherein the Fe/Sr ratio is ina range of 6 to
 20. 30. The rolled 6xxx-series aluminium alloy sheetproduct according to claim 1, wherein the Mn-content is in a range of0.02% to 0.20%.
 31. The rolled 6xxx-series aluminium alloy sheet productaccording to claim 1, wherein the Cr-content is in a range of 0.01% to0.20%.
 32. The rolled 6xxx-series aluminium alloy sheet productaccording to claim 1, wherein the Cu-content is at least 0.04%.
 33. Therolled 6xxx-series aluminium alloy sheet product according to claim 1,wherein the Cu-content at most 0.20%.
 34. The rolled 6xxx-seriesaluminium alloy sheet product according to claim 1, wherein theFe-content at least 0.20%.
 35. The rolled 6xxx-series aluminium alloysheet product according to claim 1, wherein the Fe-content at least0.22%.
 36. The method of manufacturing a rolled 6xxx-series aluminiumalloy sheet product according to claim 15, wherein the solution heattreated and quenched aluminium alloy sheet product has been pre-aged ata temperature in a range of 50° C. to 130° C.